April 2010, Helmand Province: Air Force Staff Sgt. Jonathan “Bax” Baxley is in an armored troop carrier, strapped in the gunner's turret. His fingers warm the handle of an M-240 machine gun; his eyes scan the Afghan plains for Taliban and IEDs.

Post and Courier Projects Reporter Tony Bartelme has been a finalist for the Pulitzer Prize twice in the past three years, most recently for “Storm of Money,” a series that exposed how insurers set South Carolina's unusually high property insurance rates. He also earned a national Sigma Delta Chi award for that project. He has been with the newspaper since 1990. Beginning in October, he will begin a one-year sabbatical to work on a book, “Send Forth the Healing Sun,” which grew from a Post and Courier series in 2010 about a Charleston neurosurgeon's quest to teach brain surgery in the African bush.

From his perch, Baxley sees a shift in the light, thick with dust. The late-afternoon sun has lost its punch, so now everything around his convoy is a hundred shades of orange. Looking right, Baxley sees mountains. To his left, a group of mud-brick homes, and off in the distance, a dry riverbed with a live rocket-propelled grenade that he needs to disarm.

Bad time to be out here; insurgents are fair-weather fighters. They typically strike in the cool of the morning, or when the worst of the day's heat has passed, like now. Baxley talks through a headset to a medic below who's on his first tour. Small talk. What do you think about Afghanistan? Having fun yet?

Baxley is with Charleston's 628th Civil Engineering Squad, 24 years old on this afternoon, solidly built. When he was growing up on James Island, family friends called him “Tiny” because he grew faster than his peers. But now his open and wide face makes him look younger than his age, boyish. Except for his brown eyes. They're steady, like a batter looking at a pitcher.

He'd played baseball at James Island High School, always a team player. He was the middle child in the family, the peacemaker, calm and helpful, the one who asked his mom, “Can I stir?” when she made things in the kitchen. Coaches liked his steel and sent him in to pitch when things were falling apart.

Calm on the surface, sure. But he was also the kid who waited extra at amusement parks for the roller coaster's front seat, who threw everything he had into driveway basketball with his best friend, Ryan Easterby.

Yes, he needed adrenaline and competition in his life, so after high school, when someone said he would make a good bomb hunter, his ears perked up. He talked to Air Force recruiters about EOD, explosive ordnance disposal, watched videos about EOD “operators” at work, and the fuse was lit.

You had to be smart to disable a bomb, unflinching under pressure, a team player. You had to think like a terrorist, then beat their booby traps or die trying. You had periods of waiting and tension, then bursts of activity. Just like baseball.

Except you can't drop the ball when you work around bombs. Baxley had seen the stakes on his first deployment to Iraq in 2009. He was assigned to an Army unit clearing roads of improvised explosive devices. On one mission, a troop carrier in front of him hit an IED, flipping the vehicle like a pancake, crushing and killing the soldier in the turret.

Now, a year later, Baxley is in the turret, assigned to a Marine combat unit. Around his face is a shemagh, an Afghan wrap that keeps him from choking on dust from the two trucks in front. His convoy veers off a dirt road and follows a cattle path to avoid mines. The live RPG is up ahead. Then, all at once, motion, noise, dust, smoke.

Action/reaction

It's easy to make a bomb. Just get some fertilizer, aluminum powder, gasoline and mix it in a jug. The Taliban often use yellow palm-oil containers. Now, with a 9-volt battery and wires, hook it to a small explosive to make a detonator. Close the circuit and let the chemistry begin.

The detonator fires, pumping energy toward that mix you just made. In a fraction of a second, this injection of energy causes the molecules in the jug to go berserk and generate a blast wave of gas and heat. The blast wave expands at 900 mph or more, usually too fast to see with the naked eye.

In physics, actions trigger reactions, and as the wave speeds outward, it leaves a vacuum in its wake. Air molecules fill this void, creating a powerful backward-moving force called a blast wind. Blast wave, blast wind; a punch to the front and one from behind.

Unless blocked, these blast forces easily rip apart tissues, especially something as thin and fragile as an eardrum. When they hit body organs filled with air or fluids, the effect is like someone stomping on balloons. Lungs collapse, spleens compress, bladders explode. Blast waves can even affect bone marrow.

After a blast, your body may look perfectly fine on the outside, but your insides might have been blown to pieces. The same goes for your brain, pulsating with electricity, blood and thoughts.

Then, from a hill about 500 yards away, a blast of bullets hits the convoy. Ambush. Physics also applies to war, and the Marines react by pumping waves of gunfire back at the hill. The air around Baxley is filled with noise from machine guns and the pings of bullets hitting the troop carrier, until an American helicopter gunship charges in and the insurgents slink away.

Inside the troop carrier, Baxley and the three others take stock. Baxley's ears ring but don't hurt. The IED exploded closest to the driver, underneath the front tire. But everyone is alive, though the driver has a nasty bruise on his leg where he mashed the door handle. The blast shredded the vehicle's undercarriage, but that's supposed to happen.

They were in an MRAP, a mine-resistant ambush-protected vehicle with a V-shaped hull. Like a ship plowing through water, the V shape deflects blast forces up and away from those inside. Had they been in a flat-bottom Humvee, he and his buddies probably would be dead.

They stay in the truck for about 20 minutes as the Marines set up defensive positions so Baxley can get to work. Insurgents often plant other mines around an IED, and it's Baxley's job to walk through this potential kill zone with a hand-held mine detector and make sure the area is safe.

Alone, Baxley climbs out. He can't use a bomb suit because it was blown up — probably wouldn't have used it anyway, too cumbersome. He grabs the detector and starts scanning the ground around the convoy for explosives. He's an easy target for a sniper, but he has to focus. Miss a mine and you could lose a buddy. Something has to be done, so you do it, he thinks.

The Marines cover him, training their machine guns in every direction, a 360-degree circle of fire. When Baxley finishes, he calls all clear. The first guy out of the carrier sees Baxley and motions toward his head. Blood is running down Baxley's right ear.

Invisible wounds

The human brain floats in an imperfect container. Beneath the skull, a thin cushion of cerebrospinal fluid protects your lobes from shocks. The skull and fluid cushion are good for minor blows, but if you smack your head really hard, your brain, about three pounds in all, will slosh back and forth, like a sponge in a bucket.

The brain has a forest of blood vessels, some as large as your finger, others smaller than the finest strand of hair. In a severe blow, the brain's shape will change, stretching and tearing these strands. The brain will swell, and because the skull has little spare room, the blood and inflamed brain have nowhere to go.

Its cavities fill and compress. If the swelling is severe, the brain might crush itself. As a last resort, neurosurgeons sometimes remove a piece of the skull to let the brain spill out, like dough rising from a pan. Then after the swelling has gone down, they stitch the skull back together.

The brain's container has other flaws: In some areas, it's as thin as a cracker. Worse, behind your nose are bony protrusions. Inconveniently near these bones are bundles of fibers, nerve cells and brain tissue called “white matter.”

Named because white insulation surrounds them, white matter connects different neural networks to one another, including the ones that manage judgment and memory. Cut those connections, and you might lose your ability to remember what happened yesterday, or control your anger when a car cuts you off.

A blast wave's effect on the brain has been compared to a baker rolling a pin over dough, but the damage is hidden, invisible to all but the most advanced forms of medical imagery, such as computed tomography (CT) and positron emission tomography (PET).

A brain has 100 billion neurons that are connected in thousands of networks. Advanced scans can now show how blast forces alter the brain's chemistry on a cellular level, causing neurotransmitters to flood the brain's wiring. Circuits go haywire; you feel confused, delirious. Or they shut down completely, and your mind goes dark.

A blast wave often carries with it rocks, metal and other shrapnel, and in past wars, it was these flying projectiles that did in many soldiers. Now, with advances in emergency medicine and lightweight body armor, more troops are surviving these injuries. But body armor and helmets also are imperfect protectors. They may stop bullets and shrapnel, but not the jarring forces of the blast wave itself.

Sucker punch

Four hours after the explosion, Baxley is back at Patrol Base Buji, a ring of sand barriers overlooking a putty-colored mountain range. It's in one of the most remote areas of Helmand, deep in Taliban territory. It has no significant medical equipment, but it does have a Navy medic, who checks Baxley's ears. The medic tells him the blast pressure tore his eardrums, but they'll heal.

The medic asks Baxley questions designed to diagnose a concussion. Baxley thinks he does fine on the test, but the medic thinks otherwise and tells him he has a low-grade concussion.

The next morning, Baxley wakes up feeling like a boxer has pummeled every inch of his body. He gets ibuprofen and Tylenol from the medic, but the pills barely make a dent in the pain. The medic asks him more concussion questions, then clears him for patrols the next day. That's a relief, headache or not. He's there to do a job, not sit on the bench.

Three days after his truck was blown up, Baxley is in another convoy when someone spots an IED on a curve in the road. The vehicles stop, and he unloads a 115-pound robot. Using joy sticks, he sends the robot to the IED and disarms it, textbook. But as he's loading the robot back into the truck, a driver makes a mistake; he's not supposed to move forward when people are outside their vehicles, unprotected, but he does, rolling over an IED they had missed, 50 yards from Baxley.

The blast wave hits him like a linebacker. His knees buckle. He falls to the ground, his arms and legs drained of strength. “Bax, are you all right?” someone yells.

The blast destroys the troop carrier, but once again the V-shaped hull saves everyone inside. Back at the base, Baxley goes to the medic. Another test, another concussion. “Could have been a lot worse,” the medic tells him.

Baxley isn't complaining. His head pounds, and his ears will ring for weeks. But this isn't a football game; it's an increasingly violent war in a forward base surrounded by roads filled with IEDs. You're alive. What's a headache when you've dodged bombs?

After another day off, he's back with the Marines, who have a new rule that you get extra leave when your convoy has been blown up three times in a certain time period. Marines being Marines, they root for another IED blast: “One more! One more!”

Repeated blows

Is one more blow to the head too much? You can find answers in sports, especially boxing. You can see it in a punch-drunk fighter's trembling hands, his slurring words. You could see it in Sugar Ray Robinson, perhaps boxing's greatest, who died of dementia, penniless. And in Jerry Quarry, who spent the last years of his life unable to recognize his parents or children and had a brain his neurologist compared to “a grapefruit that has been repeatedly dropped.”

Doctors had studied punch-drunk boxers since the late 1920s, but it wasn't until the early 1980s that researchers truly saw what repeated blows did to a boxer's brain. Using new CT and PET imaging machines, they scanned boxers' brains for abnormalities, which they soon found.

One researcher in 1983 took CT scans of 40 former boxers and found signs of brain damage in more than half. A 1983 report in Sports Illustrated, “Too Many Punches, Too Little Concern,” generated calls for boxing reform, and in 1984 the American Medical Association demanded an outright ban.

Amid this wave of concern, David Hovda landed a tough assignment. Hovda had founded UCLA's Brain Injury Research Center in Los Angles, and in the early 1990s, Hovda got a call from Jose Sulaiman, the head of the World Boxing Council. Sulaiman wanted to know: Was there a way to decrease brain injuries in a sport where people were paid to knock each other's blocks off?

Hovda is an imposing man, 6 feet, 2 inches tall. In junior high school he was a linebacker and sometimes fired up teammates by banging his head against lockers. Early in his career, he grew fascinated by the brain's mechanics and chemistry. Why, he asked himself, couldn't you open the skull like the hood of a car and fix the wiring?

At UCLA, he had the technology to open that hood. The clinic also happened to be across the street from the football team's practice field, and one day, the sideline doctor brought in a linebacker with a concussion. Hovda and his colleagues took PET scans of the player's brain chemistry, along with another patient in a deep coma from a car accident. When the scans were ready, someone handed him the football player's results.

“I looked at it and said, 'You gave me the wrong one,' ” Hovda recalled, thinking the scans belonged to the comatose patient. But it wasn't a mistake. The brain scans of the linebacker, who could walk and talk just fine, were remarkably similar to those of the motionless patient with the coma.

For Hovda, this was proof of something he'd seen in animal studies: In severe and relatively mild concussions, brain cells shut down and burn enormous amounts of glucose, like drained batteries getting a recharge. The good news was that the brain healed itself if you gave the cells time, as eventually happened to the football player. The bad news was that brain cells in this recharging state were particularly defenseless. Another blow to the head could knock them out for good.

Hovda's work and other emerging research about the dangers of repetitive concussions began to shape emotional debates about boxing and other sports, particularly football. As with boxers, prominent NFL players were falling apart in their retirements.

“Iron” Mike Webster had been one of the best centers in the NFL, a Hall of Famer who won four Super Bowls with the Pittsburgh Steelers. But after he retired in 1990, he lived out of his pickup truck and slept in train stations. When he died at age 50, doctors found he had chronic traumatic encephalopathy (CTE), the same disease seen in punch-drunk boxers.

Former Chicago Bears star Dave Duerson also had CTE when he shot himself in the chest in 2011, leaving a note that his brain be donated to Boston University for research.

Hovda thought this new body of evidence could help save lives, or possibly prevent former athletes from living out their retirements in a fog of dementia. He told boxing leaders how to identify fighters who had their bells rung too hard; he urged them to stop fights earlier and keep fighters from sparring too soon after tough bouts.

His work captured the attention of NFL players, and he told their doctors about the importance of keeping players with concussions off the field until their brains fully healed, even if they said they were fine. “Athletes, just like those in the military, do a good job of lying,” Hovda said. “They say, 'Sure, coach, I can go back in.' ”

Presence of abnormal

Baxley's 2010 deployment at Patrol Base Buji is a blur of combat and explosions. The war's kinetics have shifted. The coalition's troop surge is well underway, and the insurgents' reaction is to plant more IEDs than ever. They're burying them in vegetable fields, cramming them into road culverts, booby-trapping pomegranate groves; they're making new kinds of IEDs that fire jets of plasma at 5 miles per second, melting holes in armor so copper slugs can fly through.

The number of IED attacks has tripled from presurge days to more than 40 a day. On average, six troops a day are being killed or seriously injured in IED blasts.

Baxley and other troops don't talk about whether they'll get blown up, but when. And not just at Patrol Base Buji. West of Baxley's base, Staff Sgt. Douglas Ryan, another Air Force operator with Charleston's 628th EOD unit, gets blown up three times. And Ryan knows a Marine EOD operator who was disabling an IED when a mortar round landed nearby. The blast forces blew the Marine away from the IED, then a second mortar round landed by him, blowing him back toward the IED, which exploded.

Somehow the Marine survived, but Ryan remembers seeing him a week later, the stunned look still etched in the Marine's face.

Economics favor the insurgents; for less than $30, a bomb-maker can take out a $500,000 MRAP. The U.S. military is pouring more than $60 billion into mine-resistant vehicles and anti-IED technology. But technology goes only so far.

Hunting bombs is personal; you have to get inside a bomb-maker's head, study his tactics, then adjust. Some nights at Patrol Base Buji, messengers from the village come with locations of newly planted IEDs. Sometimes Baxley and other EOD operators in the region get tricked and the IEDs really are 300 yards in front of where the snitches said they would be, traps.

Baxley and others at the base constantly are on guard for the uneasy feeling that something is amiss, what his British EOD counterparts call the “presence of abnormal, absence of normal.” What was that blue jug on the road the day before? Shouldn't there be more people in the fields at this time of day? Miss a sign, and you could roll over a pressure plate. Which happens anyway. During one 10-day stretch, 13 vehicles in Baxley's unit get blown up.

Connecting the dots

Presence of abnormal, absence of normal. As Baxley and other EOD operators scoured roads for signs of bombs, researchers in Boston also were hunting for abnormalities, though their landscape was the brains of dead athletes.

For several years, Ann McKee, a professor of neurology and pathology at Boston University, had built a collection of tissues of former NFL players and other athletes, including Dave Duerson, the Chicago Bears defensive back who committed suicide when he was 50 years old.

In her lab, she stained slides of these tissues brown to bring out damage from chronic traumatic encephalopathy. This coloring helped reveal how brains with CTE had tangled strands of “tau,” a protein that helps brain cells communicate with one another.

If you had too many tangled tau proteins, then your neural networks failed. When enough of these networks went down, you showed signs of Alzheimer's and other forms of dementia.

One day, Lee Goldstein, a medical researcher at BU's School of Medicine and College of Engineering, walked into her office, saw the slides and brown stains and all but shouted, “It's the physics!”

By that, Goldstein meant that violent back-and-forth movements might be the source of these tau protein tangles, whether it was from an IED's blast or a linebacker's hit. He called it the “bobblehead effect,” and with McKee and a team of researchers, they set out to test their hypothesis.

First, they found brain tissues of four athletes in their teens and 20s with documented cases of concussions before their deaths; three played football and one was a professional wrestler. Next, they acquired brain tissues from four military veterans; three had been exposed to IED blasts and the fourth had a history of multiple concussions. Finally, they analyzed tissues from people who had no known concussions.

And there it was, in brown and white: The soldiers' and young athletes' tissues were filled with tangles of tau proteins, while the brains of those without concussions had almost none. It was the strongest evidence yet that veterans exposed to IED blast forces had the potential to develop CTE, the same condition that caused boxer Jerry Quarry and football player Dave Duerson to lose their minds.

“The point is that rather than getting hit by a 300-pound linebacker, you're getting hit by compressed gas that's going hundreds of miles an hour,” Goldstein said.

But Goldstein and McKee had more questions: Was this damage caused by something within the blast wave itself? Or was it the physics: the intense oscillation from blast forces?

To find out, they placed mice in 27-foot-long aluminum tubes and blasted compressed air at them to simulate blast forces. Some mice were strapped tight so their heads wouldn't move. Later, when they looked at the mice's brains, they found the tightly strapped mice fared well, while the others had brain injuries — the bobblehead effect in action.

Their study made international headlines when it was published in 2012. It was one of the first to link sports to IED blast forces, and soon Goldstein and McKee were getting one letter and email after another from troops and their families. “They said, 'Thank you for telling me I'm not crazy,' ” Goldstein recalled. “Parents said we knew there was something wrong with my son. He wasn't the same when he came back. Now you've explained why.”

And yet, even as Goldstein, Hovda and other researchers revealed how repeated concussions could lead to lifelong health problems, top military leaders had missed the presence of something abnormal happening to the brains of tens of thousands of troops.

The shift

In 2008, a tough-talking Army general named Peter Chiarelli was promoted to vice chief of staff, the Army's second-in-command. On the fourth day of his new job, someone handed Chiarelli a chart outlining the types of injuries in Afghanistan and Iraq. One number stood out: roughly 36 percent had traumatic brain injuries or post-traumatic stress.

“I had spent two tours in Iraq and quite frankly I didn't know what traumatic brain injury was,” Chiarelli recalled. “TBI to me was a concussion, and a concussion was my football coach telling me when I got dinged, that I should just shake it off and get back in the game and I would be fine.”

Chiarelli learned about Hovda's work, and in the spring of 2009 invited him to dinner at his home. Joining them was Gen. James Amos, then-assistant commandant of the Marine Corps. Hovda gave his pitch about the brain's vulnerability when recharging after a severe blow. He told them why it was important to take football players off the field after concussions so their brains could regain their chemical and metabolic balance. He said soldiers needed time too, or they could end up with long-term damage, just like the boxers and football players.

An explosion needs a spark, and Hovda proved to be the flint. Hovda's concussion imagery was “a real eye-opener,” Chiarelli said. “We could show a kid that his football coach was wrong, that when he was concussed, there was a change in his brain. It got at the stigma associated with something you couldn't see. Now you could see it; you could show a concussed brain, and how it shuts down.”

Moved by Hovda's warnings, Chiarelli and Amos summoned the Defense Department's top doctors, psychologists and neurologists to meet with Hovda the next day at the Pentagon. Hovda spoke for 15 minutes. Then came the blow back.

Some called his research “bad medicine.” Voices rose. One said it wasn't clear that concussions were that dangerous. Another worried that if you told soldiers they were “brain injured,” they would develop symptoms to fit the diagnoses, like hypochondriacs.

“We heard all this bickering,” said Chiarelli, who later dubbed it the “food fight.” “You would stick a hypothesis on a table with 15 people and you would get 16 answers, and someone would contradict himself by the time we got around the table. So Jim (Amos) and I stormed out of the meeting. We were disgusted.”

So disgusted that they quickly formed a panel of the nation's top traumatic brain injury experts, but allowed only one doctor from the Army to participate. Within two days, the group came up with new concussion rules: If you're within about 50 yards of a blast, or lose consciousness, you'll be screened for a concussion. Fail this test, and you'll be sent to a concussion-restoration unit.

Within months, the Army had these rules in place across the world. Other branches followed, yet the military still had plenty of ground to make up.

Special “gray teams,” named after the gray matter in the brain, were dispatched to Afghanistan, where they found that most medical workers didn't know how to properly do a basic concussion test, known as a MACE, short for “military acute concussion evaluation.”

In 2011, the military began awarding Purple Hearts for soldiers who suffered concussions but hadn't lost consciousness, paving the way for some to get VA treatments. (Previously, soldiers had to be knocked out cold.)

The military also pumped $40 million into ANAM, short for Automated Neuropsychological Assessment Metrics, a controversial computer test to identify people who have suffered concussions, a test the Army's surgeon general called no better than a “coin flip,” and Chiarelli said was “horrible, but probably the best we have.”

Last year, President Barack Obama unveiled a Defense Department and VA plan to spend $100 million on research into TBI and post-traumatic stress, a sum that didn't impress Chiarelli.

Chiarelli retired last year and now leads One Mind, a nonprofit dedicated to cures for TBI and other brain diseases. “I don't want to hear about how we're spending a hundred million dollars. It's a drop in the bucket.”

The stakes go beyond the battlefield. Every year, more than 3 million Americans suffer head injuries. Knowledge gained from new discoveries about military-related blast forces could help doctors treat children hurt in playground accidents and people with debilitating diseases, Chiarelli said. “Research is showing that TBI is connected to dementia, that it's connected to post-traumatic stress, to Lou Gehrig's disease, to Parkinson's,” he said. “The real sad part of this is that you'll be writing stories 10 years from now about the large number of people with TBI who developed early onset Parkinson's and other diseases.”

The last blast

April 2012, Baxley is deployed again in Afghanistan, his third tour in four years, and glad to be back. He's married now and loves fishing and playing softball when he's home. But like many of his fellow EOD operators, he loves the adrenaline, camaraderie and sense of purpose of destroying materials designed to hurt others.

Early in this tour, he's with a team doing a controlled blast of an IED, videotaping from a troop carrier's turret. When they detonate the IED, another one hidden 40 feet away explodes. The blast wave hits hard, and he feels a wave of disorientation.

Over their deployments, Baxley and others in EOD circles had seen the military change its stance toward concussions. “In prior deployments, the MACE test was given only for people who really had their bells rung,” Mills said. “Now, MACE is a huge thing.”

So after that hidden IED blows up, Mills pulls out a set of cards with MACE questions and gives Baxley the test. Baxley chuckles, thinking: I've been blown up multiple times; if you can ask for a MACE test, you probably don't need it. He passes, but he has headaches and nausea, as if he's tipsy and hung over all at once, though the symptoms ease after a few days.

Soon he's back in the field, and he's eager to be part of what comes next. After so many years of clearing IEDs from roads, he's with a British reconnaissance team going after the bomb-makers themselves. Instead of playing defense, they're on offense.

In the hours after midnight, they climb into helicopters that drop them into nearby villages. They do door-to-door searches for caches, then clear out as the sun rises and they lose the advantage of their night-vision gear.

On one mission they discover a bomb factory, and Baxley sees the pressure plates that detonate when vehicles roll over them, hundreds of those yellow palm-oil jugs. The team gathers evidence for military intelligence analysts, then blows the cache into the night sky.

On another mission, they're still on the ground at dawn when insurgents attack. His team takes cover behind a wall. They set up a fighting position. Suddenly, they're hit from behind. An RPG flies toward them and rams into a wall about 20 feet away. Baxley feels scattering rocks cut his hand and neck, and the pressure of the blast wave against his face.

Wave of wounds

Now it's summer, 2013. Baxley is 28 years old, back in Charleston, training at the Air Force Base and responding to occasional bomb-squad calls from local law enforcement agencies. He remembers that last RPG.

“We were behind a wall, a mud hut. A couple of those (RPGs) came through. It was a really loud crack, and I definitely felt the blast pressure. It was really hot. You have all this rapidly expanding compressed gases moving at you faster than the speed of sound, and right behind that is the negative air pressure, that actually moves backward. All that air and fluid in your body goes with it. So it feels like being hit by a train, almost a whiplash action and reaction.”

They fought their way out of trouble, but Baxley and his EOD colleagues know better than most that you can't escape physics, that when a blast wave meets your brain, something gives.

“My wife noticed something, especially when I came back after the last two deployments. My temper was a little short, but I don't know if that had anything to do with the blasts. It could just be the stress of coming back. I wouldn't say I'm worried, but I know that I've changed. My memory is worse. I couldn't tell you what I ate for breakfast yesterday, but I've found ways to cope. To remember some things, I try to remember what else happened at that time, and connect it some way. It's a lot of 'I know the answer, but I can't get it out.' The knowledge is there. It just doesn't come out as fast. I used to be awesome at 'Jeopardy.' I suck at it now.”

During his deployments, Baxley has taken out more than 100 IEDs. All told, he's disposed of more than 100,000 pounds of explosives, 15 times as much as the 1995 bomb that brought down the federal building in Oklahoma City. He's clearly had his bell rung, but he hasn't had any brain scans or been diagnosed with traumatic brain injury.

He said he took the ANAM computer test last fall after he returned from his third tour. “I failed the first one, then they called me back three days later, and it was the exact same test. I passed this time. I don't know if that had anything to do with it.”

Like many football players and athletes, Baxley and other EOD operators know how adrenaline washes through the brain when the stakes are high, that victory can be measured in inches, seconds and guts, that disarming bombs takes practice and teamwork. A lone wolf like the EOD operator portrayed in the 2008 movie “The Hurt Locker” would “never last in EOD,” he says. “But the camaraderie is amazing. You can go anywhere in the world and see an EOD tech, and you'll click, have a beer and talk for hours. And the rewards — it's the most rewarding job I've ever done.”

Since 2000, more than 270,000 service members have been diagnosed with traumatic brain injuries. Experts say thousands remain undiagnosed, and that the number may be as high as 400,000. No one's sure how many of these troops will develop long-term health problems, though Chiarelli and others expect it to be on the scale of a tsunami.

No matter the number, it's clear that Baxley and thousands of troops like him are part of a massive experiment: Many are alive despite being a few feet away from explosions that, if not for a deftly shaped piece of metal, would have killed them a few years before. And yet their brains have been subjected to abnormal forces.

“About a year ago, I got a Facebook message from the driver in the truck when I first got blown up,” Baxley says. “He's been diagnosed with a severe TBI from that blast and says he didn't have any problems until a year ago.

“He didn't hit his head, so I wouldn't say it was the blast wave that got him. It was the jarring. The explosion picked the truck up and put it down so fast, that your brain just jiggles. He had a brain scan, and now they're sending him to physical therapy, which is awesome. He's still active duty. I actually saw him for the first time since that deployment a few weeks ago, and he has a neck tic that he never had before. He was saying he had a lot of memory problems. I think everyone in this career field has those problems.”

MONDAY: The VA will absorb a wave of veterans with traumatic head injuries and post-traumatic stress: One Marine's story.

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To disarm some mines, you need to slowly find the right wire. Protecting his head and using his fingers as eyes, Air Force Staff Sgt. Jonathan Baxley clears surface dirt to find the wire during a training exercise at the Charleston Air Force Base. —Grace Beahm/Staff

Ordnance used as training tools are part of the collection accumulated in the EOD office at the Air Force base. —Grace Beahm/Staff

On a recent afternoon at the Charleston Air Force Base, Jonathan Baxley (right) was able to put his EOD duties aside and play some softball with his team, the Outlaws, with no hint that he’s been blown up multiple times. Perhaps fittingly, Baxley plays third base, also known as the “hot corner” in baseball and softball. —Grace Beahm/staff

Marines level a marketplace in Afghanistan’s Helmand Province in 2010 to clear it of IEDs. —Marine Cpl. Jeff Drew

Staff Sgt. Jonathan Baxley searches for a possible land mine as part of his stateside training with the Air Force’s Explosive Ordinace Disposal unit. —Grace Beahm/Staff

Disarming homemade bombs and IEDs requires hours of training. Staff Sgt. Jonathan Baxley searches for a dummy land mine placed in the woods at the Charleston Air Force Base. It takes him and a teammate, Senior Airman Patrick Puckhaber, about a half hour to find it and render it harmless. No live ammunition was used, but his unit tries to make scenarios as realistic as possible by tapping into the latest intelligence about insurgents’ tactics. —Grace Beahm/Staff